1. Field of the Invention
The present invention relates to display optical systems which are used in head-mounted displays and projectors for enlarged display of an original image, an moreover to optical systems which are suitable for image-taking optical systems used in digital cameras or the like.
2. Description of Related Art
Head-mounted displays, which can be worn on one's head, are one type of image display apparatus using an image forming device, such as a CRT or an LCD, for an enlarged display, via an optical system, of an original image displayed on this image forming device.
Head-mounted displays are worn on the viewer's head, so that the overall apparatus must be particularly compact and light-weight. Considering weight balance and appearance, it is preferable that they are slim with respect to the viewer's visual axis (front-to-back direction). Furthermore, in order to make the enlarged image that is displayed attractive, the enlarged image should be as large as possible.
Japanese Patent Application Laid-Open Nos. H07 (1995)-333551 (corresponds to US Patent Application Publication No. 2001-0009478), H08 (1996)-50256 (corresponds to U.S. Patent Application Publication No. 2001-0009478), H08 (1996)-160340, and H08 (1996)-179238 (corresponds to U.S. Patent Application Publication No. 2001-0009478), for example, propose image display apparatuses striving for a thinner overall apparatus, which use an LCD (liquid crystal display) as the image forming device and a thin prism as the viewing optical system.
FIG. 17 shows the optical system (display optical system) of the image display apparatus proposed in H07 (1995)-333551. In this optical system, light emitted from an LCD 111 is incident on an incident surface 113 of a small decentered prism 112. The light is bent between a total reflective surface 114 and a reflective surface 115 of the prism 112, which are curved, and then the light emerges from the decentered prism 112 through the surface 114 and is guided to the viewer's eye E. Thus, a virtual image of an original image formed by the LCD 111 is formed, and the viewer views this virtual image.
The reflective surface 115 of the decentered prism 112 is made of a decentered rotationally asymmetric surface (a free-form surface in which the optical power depends on the azimuth angle).
Moreover, the display optical system in FIG. 18 is of a type with an image forming device 101 and using a coaxial concave mirror 103 and a half-mirror 102 that is tilted 45° with respect to the optical axis of the eyeball. In the image display apparatus shown in FIG. 17, it is easier than in the apparatus shown in FIG. 18 to make the overall apparatus thin and to achieve a broad field angle of the viewed visual field.
Furthermore, an image display apparatus of high image quality has been proposed, which produces chromatic aberration that is opposite to the chromatic aberration caused by the refraction optical system and suppresses the aberrations of the overall optical system by using a diffractive optical surface within the optical system.
For example, Japanese Patent Application Laid-Open No. 2001-142025 proposes an optical system which arranges a diffractive optical surface on one surface of a prism made of three surfaces constituting an eyepiece optical system, and corrects decentration chromatic aberration while making the overall apparatus thinner.
Moreover, Japanese Patent 2705880 (corresponds to U.S. Pat. No. 5,436,763) proposes an optical system in which an intermediate image is formed by a relay optical system on which a diffractive optical surface is arranged, an eyepiece optical system having a concave mirror is arranged and decentered, and an image formed by the image forming device is guided to the viewer.
Furthermore, Japanese Patent Application Laid-Open No. 2001-004955 proposes an optical system including a prism made of at least three surfaces, in which light emitted from a light source (LED or laser diode or the like) is scanned to project an image and form an intermediate image of that image using a relay optical system on which a diffractive optical surface is arranged, and the image is guided to the viewer by an eyepiece optical system having a concave mirror that is arranged and decentered.
In recent years, the structures of image forming devices, such as LCDs and the like, have-becoming ever finer, and image forming devices have been developed which are miniaturized while having a similar number of pixels as conventionally. Using such miniaturized image forming devices is advantageous for making systems smaller, but it causes the need to increase the magnification of the optical system in order to attain the same broad field angle as conventionally.
However, when an intermediate image is formed within the optical system in order to increase the magnification, then there is the problem that the optical path length becomes long and the apparatus becomes large.